1. Field of the Invention
The present invention relates to a hydraulic machine that has a wheel mounted to move in rotation relative to a stationary structure and about a stationary axis of rotation, which wheel is designed to pass a forced flow of water therethrough. Such a flow is at a relatively high pressure that depends, in particular, on the geometry of the machine and on the general configuration of the installation of which the machine is part, in particular on the head when the machine is a turbine. The pressure of said flow generally lies in the range 3 bars to 80 bars. The forced flow through the wheel of the machine causes said wheel to be driven in rotation when the machine is a turbine. Said flow results from said rotation when the machine is a pump. The invention is applicable to hydraulic machines of the turbine type, of the pump type, or of the pump-turbine type.
2. Brief Description of the Related Art
In hydraulic machines, e.g. Francis-type water turbines, it is known that a labyrinth can be disposed between firstly a portion of the turbine that is subjected to a water pressure similar to the pressure of the flow and secondly a “low-pressure” chamber from which the water that flows through the labyrinth can be removed towards the downstream end of the installation. The flow-rate of water that passes through the labyrinth constitutes a leak, which can be non-negligible, relative to the quantity of water brought to the turbine wheel. Said leak is not a driving flow, i.e. it is not used to drive the turbine wheel in rotation about its axis. In general, a Francis turbine also has a bearing designed to center a shaft supporting the wheel relative to its axis of rotation. Usually, a shaft seal is also provided that is designed to prevent water from flowing towards an alternator or towards some other portion of the power plant. Those various items of equipment, namely the labyrinth, the bearing, and the shaft seal, are relatively costly, and each of them requires regular maintenance, which further adds to the costs of operating a prior art Francis turbine.
Analogous problems arise with pumps, in particular with centrifugal pumps, and with turbine pumps.